When an x-ray tube is being used for applications such as cardiac angiography, the x-ray tube must be operated in relatively short bursts at relatively high frequency in order to obtain clear images and to be able to monitor heart activity and detect any abnormalities. Typically, the tube would be operated in approximately 8 ms bursts at rates of 60, 30, 15, or 7.5 frames-per-second. This is especially valuable for use with television or digital imaging techniques, where the phase of the television scan is locked to the power line phase.
Since an x-ray tube typically requires in the area of 150,000 volts to be operated, a high tension power supply utilizing a step-up transformer is required to raise available AC line voltages to this level. While rectification may occur in the primary circuit of such a transformer, the phase of all of the signals being applied to the transformer being of the same polarity would result in saturation of the high tension power transformer core, preventing the power supply from producing the desired outputs.
Therefore, since, in order to achieve constant phase of the exposures, it is necessary that the half cycles on which the x-ray tube is triggered be of the same polarity, an open bridge rectifier has been employed in the secondary of the transformer in order to achieve pulses of the desired polarity and phase to trigger the tube. If triggering of the tube at less than 60 frames per second is desired, circuitry may be added to selectively pass the input pulses to the transformer in order to achieve the desired frame rate.
However, when the power supply is operated with such an open high voltage bridge, the primary circuit is under load only during the half cycle when the x-ray tube is being operated and current is being drawn in the secondary, the open bridge effectively acting as an open circuit during the other half cycle. Since there is almost no line current during the other half cycle, there is little line voltage drop during this half cycle resulting in higher average voltage levels. This results in the time integral of the voltage applied to the transformer having a net DC component in the unloaded direction (for example, in the negative direction) resulting in the transformer core saturating in this direction. As previously indicated, such saturation prevents the power supply from generating the desired output and may prevent it from generating any output.
Heretofore, problems of this type have been dealt with by using a high frequency microprocessor controlled design with a high voltage semiconductor compensated network and long down, or fall times, or with cores having air gaps to prevent saturation, or with specially designed, very expensive transformers. While in some instances these prior art systems have provided satisfactory results, such equipment has been expensive, in some instances failure prone, and the results have not always been satisfactory. A need therefore exists for a simple, inexpensive, reliable and effective means of preventing transformer saturation in a high tension power supply utilizing transformer coupling.